The study aims to better understand the sources and solutions of nitrogen pollution

Blue-green algae bloom on the surface of the water due to runoff from fertilized agricultural areas and sewage effluents rich in phosphorus and nitrogen which promote algae growth.

Study traces source of nitrogen pollution affecting world’s second largest barrier reef

Farms are responsible for the vast majority of nitrogen pollution pouring into the Mesoamerican reef region, concludes an August article in the journal Oceans and Coastal Management. Knowing where the pollution comes from, the researchers say, will help managers tailor mitigation solutions.

Tropical coastal ecosystems are among the most biodiverse areas on the planet. And they are also on the front line of effects caused by human activity. This is why it is becoming increasingly important, especially as the human population grows, to manage the impacts of runoff and sewage flowing into the sea.

“Tropical coastal ecosystems, such as coral reefs, are oligotrophic, meaning they are located in nutrient-poor waters and have therefore adapted to these conditions,” said Madeline Berger, researcher at the National Center for Ecological Analysis & Synthesis (NCEAS) from UC Santa Barbara. ). “An increased supply of nutrients can therefore disrupt the functioning of the ecosystem.”

“Our study highlights that different management strategies will need to be used in different watersheds to help reduce the input of nutrients that can adversely affect the health of coral reefs and seagrass beds in this area,” Berger said.

The Mesoamerican Reef (MAR) is the largest barrier reef in the Western Hemisphere, second only to the Australian Great Barrier Reef in size. Shared by Mexico, Belize, Guatemala and Honduras, it stretches nearly 700 miles and is home to a variety of creatures, including hundreds of species of fish, critically endangered sea turtles, as well as molluscs, marine mammals and shorebirds. These habitats and communities, in turn, support local fishing as well as tourism. Millions of people flock to the resorts and ports of Cancun, Cozumel, Belize City and other points along the coast each year while vacationing in the Caribbean.

But the reef is also in trouble. More than half of it is in disrepair due to various threats, including warming oceans, unsustainable fishing and pollution.

“Nutrient pollution is a known threat to coral reefs and seagrass beds,” Berger said. Too much nitrogen, a common pollutant, leads to a chain of events that results in oxygen-free “dead zones” and acidic conditions that can severely weaken or kill fish and other animals. Water quality issues are also among the main reasons for sindroma blanco, a pathogenic disease that results in white lesions that spread onto coral, leading to death in just a few weeks. Nutrient pollution in this region, the researchers say, “comes from four sources: agricultural production, human sewage, atmospheric deposition, and wildlife feces,” with agriculture and human sewage suspected. to contribute most of the pollution.

To tell how much of the pollution came from upstream agriculture or human sewage, the researchers dove into several models looking at the watersheds that feed into the MAR. One was a global wastewater model that estimates the amount of nitrogen pollution based on population maps, protein consumption and known rates of nitrogen excretion by humans, which has been adapted to this specific region.

“Another NCEAS group was also creating a spatially explicit global model quantifying the ecological footprint of food production, so we saw an opportunity to synthesize the two models to compare wastewater nutrient pollution to nutrient pollution. from plant and animal production,” Berger said.

The researchers also modeled the impacts of the millions of tourists traveling the coast using hotel location data, cruise ship location data and monthly statistics reported by each country’s tourism bureaus. “Basically, we were trying to figure out where the tourists were pooping, which turns out to be quite tricky,” she said.

The group found that agriculture was by far the largest contributor (92%) of nitrogen to MAR, due to runoff from fertilizers and livestock waste that goes to the ocean via rivers and streams. Two rivers, the Rio Ulua in Honduras and the Rio Motagua in Guatemala, contributed more than 50% of the modeled nitrogen pollution, collecting runoff from several upstream tributaries and discharging into the ocean. Meanwhile, 90% of the modeled nitrogen pollution has been attributed to 20 (out of 430) watersheds, 11 of which are located in Guatemala or Honduras. An estimated 80% of coral reefs and 68% of seagrass beds have been exposed to nitrogen pollution from watershed plumes.

The results are indicative of the complexity of the pollution problem – even land use changes and agricultural expansion far inland can have impacts on marine ecosystems, Berger said. Additionally, the researchers found that habitats with high biodiversity were more exposed to plumes from smaller coastal watersheds. And while tourism accounted for a very small proportion of overall pollution, the watersheds that contributed the most to pollution also tended to have high numbers of tourists or be close to those who did.

“The exercise also raised the question of how tourists, or tourism demand, have larger impacts beyond just the physical presence of more people – such as increased construction or expansion of agriculture – which can also exacerbate pollution input,” Berger said. The researchers hope that ultimately a granular approach that can be used to track the origins and trajectories of nitrogen and other nutrient runoff and their impacts will allow coastal managers to develop the sophisticated action plans needed to mitigate pollution in some of the world’s most biodiverse coastal areas. Regions.